How Would You Approach 1.6mil Unmarketed Email List?

Don't even go down this road.
8 months is too long for a list to stay dormant, let alone 8 years.
This list, being 8 years old, will be filled with inactives, bounces, traps, and a bunch of other garbage.
Your just asking for a ton of problems, especially if you are not a veteran email marketer.
Just my two cents...

I'd throw it away, not believing for a moment that it could possibly have any value to me, while the huge potential disadvantages of trying to use it (which include, among others, all the problems discussed in this post and in the other threads linked to, there) are all-too-readily foreseeable.

They'd be "remarkable circumstances indeed" that made it legitimate for you to use the list, anyway: for a start, all the potential recipients would have to have given permission, at the time they opted in, for subsequent third-parties to contact them, and so on.

I'd expect something like a 1% open-rate and 0.01% click-through rate, and a flood of spam complaints.

However, John's response just above is undeniably far more constructive than mine, so I wouldn't blame you for preferring that one.

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I agree with what most are saying in here ... Dump it. NOT worth the hassle, and you will run into more problems than you can imagine, even after cleaning it.

I would not dump it.

Those are customers who spent money at some time, given that it is travel clients each potentially cost of couple thousand dollars, even at 2-3% open rate you looking on big potential profit.

Just make sure you split this list in much smaller chunks and don't send email to all of them at once.

One thing in being successful business person is to be able take any opportunity that comes your way and find a way to monetize it, as long as the opportunity is legal.

Would You not just send the list a message asking them reconfirm their subscription, that way you have a new list that want good content

First things first, clean the list (there are services out there that do this - I can recommend a few if you want). If your bounce rate is too high, your message will never make it to the rest of the list. I'm in a similar situation right now, have about 800k emails from an old brand that has never marketed to them - after cleaning the list only about 300k of those email addresses are still active.

Start there and see what list size you're working with.

Eight years is a long time ... I would start with an announcement, not a marketing message. Announce a newly redesigned website, company ownership change, etc. Remind them somewhere in this email that they are a CUSTOMER - don't just hammer them with an ad out of the blue since they may not remember purchasing from you.

Let everyone who plans to unsubscribe do so. It will probably be significant. Re-mail the non-opens at least one more time.

Then switch list providers, and don't port over the unsubscribes. That way when you really start to get into the marketing, you're working with a list of subscribers who have accepted you and won't be nailed with any bounces or unsubscribes with your new provider.

Thanks a bunch to everyone who answered! Extremely insightful responses.

To be clear: this was 1.6 million paying customers to the clients business of 15 years. He has worked with other channels in the past but wants to break into email now. He was collecting emails when people created accounts/bought from him but wasn't engaging the list.

I'm planning on taking ronrule's advice above. Great stuff!